High voltage power supplies are, among other things, used for high voltage energization of electrostatic precipitators. An electrostatic precipitator is used e.g. in filtering particulate in exhausting gases from industrial processes. Often electrostatic precipitators comprise a number of fields in series in the gas direction. Because the dust concentration decreases along the electrostatic precipitator, i.e. the dust concentration at the inlet of each field is different, each of them typically has its own high voltage power supply.
The main unit in such high voltage power supply is a so-called transformer-rectifier set (TR set) comprising a high voltage transformer and a high voltage bridge rectifier. Transformer-rectifier sets can be single-phase or three-phase depending on the particular application of the electrostatic precipitator, and they are often immersed in a transformer oil filled tank.
The power delivered by the transformer-rectifier set to the electrostatic precipitator may be regulated by controlling its primary side by a semiconductor switch controller, e.g. a thyristor controller, that may be mounted inside a control cabinet. The thyristor controller comprises a pair of thyristors connected in antiparallel and is sometimes also called an AC line regulator. Alternatively, a semiconductor switch controller utilizing other types of controllable semiconductor switches may be used. This controller uses the principle of phase control for varying continuously the power delivered to a load. Phase control means that the firing angle (i.e. the phase angle at which e.g. a thyristor is fired or triggered in a given half period of the line frequency) of the individual switching elements, e.g. the thyristors, may be delayed/increased (i.e. fired later) for decreasing the power delivered to the load or it may be advanced/decreased (i.e. fired earlier) for increasing the power delivered to the load. The controller and the transformer-rectifier set may be protected by a circuit breaker and may be connected and disconnected by means of a main contactor.
The firing angle is normally determined in an automatic control unit that may be microprocessor-based and then transmitted to firing circuitry, where the firing command is converted into two firing pulses 180° apart having the correct width, which are then applied to the gate of each thyristor or other type of switching element.
In order to get the best efficiency of the electrostatic precipitator, the voltage applied to each electrostatic precipitator field should be as high as possible. The limiting factor here is the breakdown of the gas treated by the electrostatic precipitator in the form of sparks or arcs that may occur at high voltages. The difference between sparks and arcs is the duration of the breakdown. A spark is very short, while the electrostatic precipitator voltage in case of an arc remains low as long as the surge current is present, which may be for several half periods of the line frequency.
After a breakdown, the electrostatic precipitator voltage must be recovered by firing the switching controller again in order to ensure an efficient capture of particulate. Thus after the surge current has elapsed, a firing angle for the switching element has to be determined, so that the electrostatic precipitator voltage can be recovered as fast as possible. However, if a high voltage level is attained too fast, it may cause multiple sparking, i.e. new sparks may occur in the recovery period, which is detrimental for the efficiency of the electrostatic precipitator. On the other hand, a too slow recovery is also detrimental for the efficiency of the electrostatic precipitator.
The sparking level depends mainly on the gas composition, temperature and humidity, and the dust concentration as well. Thus the sparking level is not constant, and therefore, a quite common procedure is to reduce the voltage level after a breakdown by selecting a later firing angle than before the breakdown and then advancing the firing angle gradually for increasing the electrostatic precipitator voltage until a new spark occurs. This means that the transformer-rectifier set is operated at a certain spark rate, commonly in the range 10-60 sparks/min.
Examples of systems using this solution are known e.g. from U.S. Pat. Nos. 4,860,149 and 5,689,177. In U.S. Pat. No. 4,860,149, the power is, to avoid the risk of multiple sparking, immediately after the spark reduced to zero where it remains for a period of time (blocking period) of up to 50 ms. The power or the voltage is then increased along a relatively fast ramp from zero to a setback level (at a certain percentage below the level before the breakdown) over a time period that may also last several half periods of the line frequency. The power or the voltage is then gradually increased along a slow ramp until a new breakdown occurs. This solution very well reduces the risk of multiple sparking, but the blocking period and the ramping up of the voltage from zero results in a slow recovery of the electrostatic precipitator voltage, which is detrimental to the efficiency of the electrostatic precipitator. Further, all control actions are based on the primary current and the output current delivered to the electrostatic precipitator, which impairs the voltage recovery considerably.
In U.S. Pat. No. 5,689,177, the frequency of breakdowns, i.e. the spark rate, is minimized by first quenching the breakdown in N half periods and then controlling the firing angle by means of three ramps whose slope is determined by statistical calculation based on data, where previous firing angles seem to be the most important parameter. It is noted that in this document the term “firing angle” is used in the meaning “conduction angle”, which is in contrast to the present application. This method has a shortcoming as the control process is initiated by introducing a quench or blocking interval of N half periods, where the output power delivered to the electrostatic precipitator is zero. Together with the ramping up of the voltage from zero, it results in a slow recovery of the electrostatic precipitator voltage, which is detrimental to the collecting efficiency of the electrostatic precipitator. Alone the presence of the blocking period can cause a decrease in the average voltage applied to the electrostatic precipitator field of as much as 5-6 kV.